The BEGIN/END block in
the THEN part defines a child parsing
context (let's call it 'A'), that contains local variables
v1 and v4,

Likewise, the ELSE block defines a
parsing context (let's call it 'B') which is a child of the
root, and contains local variables v2 and
v4.

The total number of symbols is 9: 5 for the root + 2 for A + 2
for B. All the symbols are numbered internally (starting at
offset 0), by walking the parsing context tree in a depth first
manner, resulting in the following: